Development of a specific radioimmunoassay for vitamin B₁₂ and its application in the diagnosis of cobalt deficiency in sheep

A radioimmunoassay (RIA) for vitamin B₁₂ is described. Antisera were raised in rabbits using a conjugate between 5′-O-succinyl cyanocobalamin and chicken serum albumin. The antibody is largely specific for the β (upper) face of the vitamin B₁₂ molecule. The RIA can detect vitamin B₁₂ down to concentrations of 37 pmol/L and correlated well (r=0.980, p<0.001) with a commercial radioassay. The use of this RIA, together with an assay for plasma methylmalonic acid, in the diagnosis of cobalt/vitamin B₁₂ deficiency in sheep is described.